According to the UK Home Office:
Regulation 11(4) states that when an EEA national does not produce on arrival a valid ID card or passport he should be given “every reasonable opportunity” to prove by other means that he is an EEA national. In other words a person claiming to be an EEA national should not automatically be refused admission as a result of being unable to produce a valid ID card or passport.
In practice, what documents can be used?
I am a citizen of an EU country (recently naturalised), but do not yet possess a passport or ID card of that country due to exorbitant processing times.
If I were to show up at the UK border via the crossing from France with my certificate of citizenship (stating my full name & date of birth), and a foreign passport (verifying my name & DOB), would this constitute sufficient evidence? Would they merely call the embassy to confirm, or in practice would I be denied entry?